In air-tunnel studies it is often necessary to measure the absolute magnitudes of pressure and velocity or to conduct tests at a given Reynolds number. For this purpose it is important to determine precisely the density of the air in the tunnel as a function of temperature, humidity, and barometric pressure. Because of the laborious evaluation involved, the influence of the latter two quantities is usually neglected, despite the fact that it is of the same order of magnitude as the influence of temperature. In the following a nomogram is developed which permits determination of the air-density without time-consuming calculation. The effect of density changes upon pressure and velocity measurements is illustrated by a factor C for the instrument most commonly employed in air-tunnel studies: a Pitot tube as the sensing element and a differential manometer as the recording device. An analogous factor could be derived for other measuring techniques.